Gain stabilizer for an electron multiplier tube



Sept. 30, 1958 c. F. ROBINSON GAIN STABILIZER FOR AN ELECTRON MULTIPLIER TUBE Filed Aug. 27, 1956 United States lPatent GAIN STABILIZER FOR AN ELECTRON MULTIPLIER TUBE Charles F. Robinson, Pasadena, Calif., assignor to Consolidated Electrodynamics Corporation Application August 27, 1956, Serial No. 606,310

9 Claims. (Cl. Z50-41.9)

This invention relates to electron multiplier devices and, more particularly, is concerned with the stabilization of gain of an electron multiplier device used in measuring ion currents for a mass spectrometer.

The use of electron multiplier devices to measure small currents is well known. Thus, in an electron multiplier tube or a photomultiplier tube, a plurality of electrodes are provided, and operated at potentials which accelerate electrons from one electrode to the next. Due to secondary emission effect, each electron striking an electrode dislodges a number of electrons which, in turn, are accelerated to the next electrode. If the number of secondary electrons is k times the number of primar3 electrons, then the gain of the multiplier tube having ten stages is klo. Since k may be of the order of 2 to 4, the overall gain may be, for example, of the order of 103 to l0D for a ten-stage multiplier. Since k varies as a function of the electron energy, and therefore varies as a function of the accelerating potentials between the electrodes, the gain can be adjusted through a very wide range by controlling the potentials applied to the electrodes.

Because of the large current gain which can be developed by electron multipliers, their use is particularly suited to mass spectrometry for measurement of small ion currents. However, the electron multipliers heretofore have been limited in their value for use in quantitative work since their gain is a Very sensitive function of the secondary emission coeicient of the surfaces which, in turn, is affected by exposure to air, water vapor or other gases, and by aging and other causes not fully understood. Furthermore, the number of electrons ejected from the lirst electrode surface by an incident ion has been found to depend not only on the ion energy but also on the chemical properties of the ion.

The present invention provides means for stabilizing fthe gain of an electron multiplier to correct for the efects mentioned above. The stabilizer provides a substantially constant gain regardless of changes in the secondary emission of the electrodes due to any cause, including changes in the specific emission coefficient of the primary or first electrode associated with various kinds of ions, both as to their energy and their chemistry.

ln brief, the present invention provides an electron multiplying device for measuring current in an ion beam from a mass spectrometer or the like. A variable voltage supply for the electron multiplier tube includes a voltage divider on the output for supplying successively higher voltage to the successive electrodes in the electron multiplier tube. A D.C. amplifier is coupled to the output of the electron multiplier tube. Also coupled to the output of the electron multiplier tube is pulse rate responsive means for producing a D.C. voltage that varies in proportion to the pulse rate of the electron multiplier tube output. A bridge circuit is connected across the output of the D.C. amplifier and the pulse rate means for producing a voltage that varies in magnitude and polarity with changes in the ratio of the output voltage ice of the D.C. amplifier and pulse rate means. This voltage developed by the bridge circuit is used to control servo means for changing the output of the variable voltage supply. In addition, means is provided for disconnecting the servo means when the count rate, as indicated by the pulse rate means, exceeds a predetermined level.

For a better understanding of the invention reference should be had to the accompanying drawing, wherein:

Fig. l is a schematic block diagram of the electron multiplier gain stabilizer circuit of the present invention;

Fig. 2 is a graphical plot of a typical mass spectrometer output of ion current as a function of mass number and is useful in explaining the operation of the present in.

vention; and

Fig. 3 is a graphical plot of the output of the pulse rate circuit as a function of pulse input rate.

ln the drawings, the numeral 10 indicates generally a mass spectrometer which may be of the type described in Patent No. 2,551,637. The mass spectrometer generates an ion beam, the number of ions, and hence the ion current, appearing as a function of the mass number of the ions present according to the sample of material being tested. This is shown graphically in Fig. 2. A mass spectrometer provides a means for identifying a mass number and must include means for measuring the ion current for each of said mass numbers. Since the ion current is relatively small, some current amplifying means is desirable in measuring the ion current. An electron multiplier tube is generally used for this purpose since it provides a high order of gain which is uniform over a broad band of frequencies and which is quite insensitive to electrical pick-up; yet such tube has essentially zero intrinsic noise or zero drift if surfaces of high work function are used for the electrodes. Y

Since a vacuum seal is generally not permeable to ions,

the electron multiplier tube is made part of the evacuated system of the mass spectrometer. Thus, as shown in Fig. l, the electron multiplier tube is provided with an envelope 12 which is part of the evacuated system of the mass spectrometer 10. The electron multiplier tube includes a plurality of electrodes, sometimes referred to as dynodes, such as indicated at 14, 32, there being ten shown in the figure by way of example. The rst electrode 14 acts as a cathode against which the ion beam impinges, releasing secondary electrons which are accelerated toward the next electrode 16. Secondary electrons from the electrode 16 are accelerated toward the electrode 18, and so the electron beam picks up density and hence is increased in current from electrode to electrode until it nally impinges on the plate 34.

The required potentials on the electrodes 14, 32 are derived from a variable potential source indicated as a battery 36 and a potentiometer 38. The voltage derived from the potentiometer 38 is applied across a voltage divider 4t) to which the successive electrodes 14, 32 lare connected. The output current from the plate 34 develops a potential across a resistor 42, which potential is applied to the input of a D.C. amplier 44. The output from the amplifier 44 is used to drive a suitable oscillograph 46 by means of which the variations in io current may be recorded.

The difficulty with using an electron multiplier tube in quantitative work heretofore has been that the gain is extremely sensitive to changes in the secondary emission coefficient of the electrode surfaces. As pointed out above, such changes are affected by exposure to air, water vapor, or other gases, and by aging in general. Since the electron multiplier tube must be made part of the evacuated system of the mass spectrometer, it is particularly susceptible to these effects.` Moreover, as pointed out heretofore, the gain of the electron multiplier tube is dependent on the chemical properties of the ions in ad- 3 dition to the energy of the ions which it is desired to measure.

Where the ion current is very low, the inaccuracies present in directly measuring current can be avoided by actually counting output pulses associated with each ion as it strikes the cathode of the electron multiplier tube. Assuming that each ion has sutiicient energy to produce at least one secondary electron, an output pulse will be produced by the electron multiplier tube which can be counted. The relation between the current at the plate 34 and the number of counts per second of pulses appearing at the output of the plate 34 is given by the following expression:

Current at anode: 1.6 X -19 X gain X counts per second l The above equation is not a rigorous expression, but 1s merely a general expression for defining what is meant by multiplier gain. It is based on the assumption that incident ions are singly charged, which is generally though not always true. The number yof secondary electrons formed in the cathode 14 by an incident ion depends on the intrinsic physical and chemical properties of the ion and also on the ion energy. In addition to the dependence of the secondary emission coeiiicient on the properties of the ion, the secondary emission coelicient at each of the electrodes of the multiplier tubes exhibits statistical variations due to variations with position of the striking electrons over the extent of the surfaces of the electrodes. All of these factors are involved in establishing the gain of the tube.

To a first order approximation then, based on the above equation, for input ion currents of l.6 10-13 amp., the counting rate is of the order of 106 counts per second, which is about the limiting rate for both practical and theoretical reasons. On the other hand, if it is desired that the count be delivered with 1% accuracy in a reading time tof 0.2 second, the lower limit of current to be measured, as set by the statistical nature of the current itself, is approximately 3.6 l0*15 amp. so that such a system can have a dynamic range of no more than 30:1 unless objectionably long reading times are allowed.

According to the present invention the output of the electron multiplier tube is applied to a pulse rate circuit 48 which is of conventional design and generally includes a clipper for limiting the amplitude of the input pulses to a xed value, and an integrating circuit for generating a voltage proportional to the rate of the input pulses. The output versus input function of the pulse rate circuit 48 is shown in Fig. 3. Such a pulse rate circuit provides a substantially linear relationship between the output voltage and the input pulse rate up `to a point at which a suihcient number of pulses appear in pairs which are too closely spaced to be resolved by the pulse counting circuit. This results in a gradual flattening out of the slope of the curve in the manner shown in Fig. 3. The maximum counting rate within the linear region is of the order of 106 counts per second.

It will be seen that if a simultaneous measurement is made of the pulse rate by the pulse rate circuit 48, and of the output current of the multiplier tube as by the D.C. amplifier 44, the ratio between the two resulting voltages is proportional to the gain of the electron multiplier tube. This is shown by the above-indicated relationship between the anode current and the counts per second. Thus, it will be apparent that the output of the D.C. amplifier 44 and the pulse rate circuit 48 may be used to stabilize the gain of the electron multiplier tube.

This is accomplished by connecting the respective outputs `of the D.C. amplitier 44 and pulse rate circuit 48 across two adjacent arms of a resistance bridge circuit indicated generally at 50. A D.C. error voltage is developed across the diagonal of the bridge circuit 50, which error signal is applied to a chopper-modulator circuit indicated generally at 52. The latter, by means of a synchronous chopper 54 driven from an alternating current reference source (not shown) produces an output at the secondary of a transformer 56 which varies in arnplitude and phase with changes in magnitude and polarity of the error voltage derived from the bridge circuit 5). The output of the chopper modulator S2 is amplified by a suitable A.C. power ampliiier 58 and connected to one phase winding of a two-phase servomotor 60. The second phase winding of the servomotor 60 is coupled to the alternating current reference source by means of a capacitor 62, which provides the proper phase quadrature relationship between the voltages on the respective phase windings of the servomotor 60. Controlling an A. C. servomotor from a D. C. error signal by means of a chopper-modulator circuit is a well-known technique, as shown by Patent No. 2,423,540 to W. P. Wills.

The servomotor 66, in turn, is used to control the potentiometer 38 by means of which the potentials on the electrodes of the electron multiplier tube are controlled to modify the gain in a manner to maintain the ratio of the output of the D.C. amplifier 44 and the pulse rate circuit 48 at a predetermined value. A zero setting is established by adjustment of one arm of the bridge S0, which may comprise a potentiometer 64, as shown.

While the above circuit as thus far described provides stabilization of the gain of the electron multiplier tube so that the output of the D.C. amplifier 44'is maintained in calibration, voperation of the circuit is limited to the dynamic range of the pulse rate circuit 48. One important feature of the present invention is to provide a stabilized circuit which is operable over a much greater dynamic range than is provided by merely providing a measurement on the basis of ion pulse rate. To this end the output of the pulse rate circuit is applied to a relay 66 which is biased to break the contacts of a relay switch 68 when the output of the pulse rate circuit 48 reaches the maximum extent of its linear range. The relay switch disconnects the servomotor 60 from control by the bridge circuit 50. Thus, the gain stabilizing function is only eiected during intervals in which the ion current drops below a predetermined level, which level is roughly of the order of 10-14 amperes. Whenever ion current peaks exceeding this level occur, the relay 66 is actuated, disconnecting the gain control loop. However, since the time interval in which the peaks of current exceed the level at which the gain stabilization is cut out are relatively short, and since the gain drift of the electron multiplier tube is generally of much longer duration, the gain is sufficiently stabilized at all times to give reliable quantitative current measurements at the output of the electron multiplier tube.

From the above description it will be seen that improved apparatus is provided for stabilizing the gainof an electron multiplier tube for use in making quantltative measurements of mass spectrometer ion currents. While the invention has been particularly described in connection with measurements of the ion currents of a mass spectrometer, it will be apparent that the same principles of the invention can be applied to other types of similar apparatus, such as a scintillation counter utilizing a photomultiplier tube. Gain stabilization of a current amplifying device can be provided whenever the individual energy pulse rate of the discrete input charge units is low enough to be directly counted by a suitable pulse rate counting circuit.

What is claimed is:

l. Apparatus for measuring the ion beam current of a mass spectrometer, comprising an electron multiplier tube including a plurality of secondary electron emissive electrodes, a first one of said electrodes being in the path of the ion beam, a variable voltage supply source including means for applying electron accelerating potentials between sucessive electrodes in the multiplier tube, means coupled to the last electrode of the multiplier tube for generating a first voltage proportional to the electrodecurrent of the last electrode, means for in'- dicating variations in the magnitude of said lirst voltage, means generating a second voltage proportional to the rate of current pulses produced by the incident ions at the output of the multiplier tube over a portion of the range of ion currents to be measured, a bridge circuit having the respective irst and second voltages applied across adjacent arms of the bridge, servo means responsive to the voltage appearing across the diagonal of the bridge including the common junction between said adjacent arms of the bridge, said servo means including a modulator and an A.-C. servo motor, means actuated by the servo motor for varying the voltage of the variable voltage supply source, whereby the gain of the multiplier tube is controlled to maintain a predetermined ratio between said first and second voltages, relay means, and means responsive to the second voltage for actuating the relay means and disconnecting the servo means when the second voltage reaches a predetermined level corresponding the maximum voltage within the linear range of the pulse rate responsive means, whereby the supply voltage control is interrupted when the ion current exceeds a predetermined level.

2. Apparatus for measuring the ion beam current of a mass spectrometer, comprising an electron multiplier tube including a plurality of secondary electron emissive electrodes, a iirst one of said electrodes being in the path of the ion beam, a variable voltage supply source-including means for applying electron accelerating potentials between successive electrodes in 'the multiplier tube, means coupled to the last electrode of the multiplier tube for generating a lirst voltage proportional to the electrode current of the last electrode, means for indicating variations in the magnitude of said irst voltage, means generating a second voltage proportional to the rate of current pulses produced by the incident ions at the output of the multiplier tube over a portion of the range of ion currents to be measured, means for deriving an error signal proportional in magnitude and polarity to'changes in the ratio of the lirst and second voltages with respect to a predetermined value, servo means responsive to the error voltage for varying the voltage of the variable voltage supply source, whereby the gain of the multiplier tube is controlled to maintain a predetermined ratio between said irst and second voltages, relay means, and means responsive to the second voltage for actuating the relay means and disconnecting the servo means when the second voltage reaches a predetermined level corresponding the maximum voltage within the linear range of the pulse rate responsive means, whereby the supply voltage control is interrupted when the ion current exceeds a predetermined level.

3. Apparatus for measuring the ion beam current of a mass spectrometer, comprising an electron multiplier tube including a plurality of secondary electron emissive electrodes, a lirst one of said electrodes being in the path of the ion beam, a variable voltage supply source including means for applying electron accelerating potentials between successive electrodes in the multiplier tube, means coupled to the last electrode of 'the multiplier tube for generating a first voltage proportional to the electrode current of the last electrode, means zfor indicating variations in the magnitude of said first voltage, means generating a second voltage proportional to the rate of current pulses produced by the incident ions at the output of the multiplier tube over a portion of the range of ion currents to be measured, means for deriving an error signal proportional in magnitude and polarity to changes in the ratio of the first and second voltages with respect to a predetermined value, servo means responsive to the error voltage for varying the voltage of the variable voltage supply source, whereby the gain of the multiplier tube is controlled to maintain a predetermined ratio between said first and second voltages, and means responsive to the second voltage for disconnecting the servo means when the second voltage reaches a predetermined level corresponding the maximum voltage within the linear range of the pulse rate responsive means, whereby the supply voltage control is interrupted when the ion current exceeds a predetermined level.

4. Apparatus for measuring current in an ion beam from a mass spectrometer or the like, comprising an electron multiplier tube coupled to the ion beam, a variable voltage supply for the electron multiplier tube, a D.C. amplier coupled to the output of the electron multiplier tube, means coupled to the output of the electron multiplier tube for producing a D.-C. voltage that varies in proportion to the pulse rate of the electron multiplier tube output, a bridge circuit connected across the output of the D.C. amplier and the pulse rate means for producing a voltage that varies in magnitude and polarity with changes in the ratio of the output voltages of the D.C. amplifier `and the pulse rate means, servo means for changing the output of the variable voltage supply in response to changes in the output derived from the bridge circuit, means for disconnecting the serv'o means when the count rate exceeds a predetermined level, and means for indicating the variations in output of the D.C. ampliiier.

5. Apparatus for measuring current in an ion beam from a mass spectrometer or the like, comprising an electron multiplier tube coupled to the ion beam, a variable voltage supply for the electron multiplier tube, means for producing a first voltage proportional to the output current of Ithe multiplier tube, means coupled to the output of the electron multiplier tube for producing a second voltage that varies in proportion to the pulse rate of the electron multiplier tube output, a bridge circuit for producing a voltage that varies in magnitude and polarity with changes in the ratio of Isaid iirst and second voltages, servo means for changing the output of the variable voltage supply in response to changes in the output derived from the bridge circuit, means for disconnecting the servo means when the count rate exceeds a predetermined level, and means for indicating the variations in the first voltage.

6. Apparatus for measuring current in an ion beam from a mass spectrometer Vor the like, comprising an electron multiplier tube coupled to the ion beam, a variable voltage supply for the electron multiplier tube, means for producing a iirst voltage proportional to the output current of the multiplier tube, means coupled to the output of the electron multiplier tube for producing a second voltage that varies in proportion to the pulse rate of the electron multiplier tube output, means for producing a voltage that varies in magnitude and polarity with changes .in the ratio of said first and second voltages, servo means for changing the output of the Variable voltage supply in response to changes in the output derived from the ratio voltage producing means, means for disconnecting the servo means `when the count rate exceeds a predetermined level, and means for indicating the variations in the rst voltage.

7. Apparatus for measuring current in an ion beam from a mass spectrometer or the like, comprising an eleotron multiplier tube coupled to the ion beam, a variable voltage supply for the electron multiplier tube, means for producing a first voltage proportional to the youtput current of the multiplier tube, means coupled to the output of the electron multiplier tube for producing a second voltage that varies in proportion to the pulse rate of the electron multiplier -tube output, means for producing a voltage that varies in magnitude and polarity with changes in the ratio of said iirst and second voltages, servo means for changing the output of the variable voltage supply in response to changes in the output derived from the rati'o voltage producing means, and means for disconnecting the servo means when the count rate exceeds a predetermined level.

8. Means for stabilizing a current amplifying device having intrinsic gain variations, the gain of the amplifying Adevice being controllable by an applied potential, comprising means for deriving a rst voltage proportional to the output current of said device, pulse counting means for deriving a second voltage proportional to the rate that discrete units of charge are applied to the input of the current amplifying device, means for varying the potential applied to the current amplifying device in response to variations in the ratio of the first and second voltages from a predetermined value, and means for interrupting said potential varying means when tbe rate of discrete units of charge lapplied to the input of the current amplifying means exceeds a predetermined value.

9. Means for stabilizing a current amplifying device having intrinsic gain variations, the gain of .the amplifying device being controllable by an applied potential, comprising means for deriving a first voltage proportional to the output current of said device, pulse counting means for deriving a second voltage proportional to the rate that discrete units of charge are applied to the input of the current amplifying device, `and means for varying the potential applied to the current amplifying device in response to variations in the ratio of the first and second voltages from a predetermined value.

2,412,423 Rajchman et al. Dec. l0, 1946 Wiley Sept. 11, 1956` 

